The present invention relates to a method and a device for controlling the braking system of a vehicle.
European Patent No. 375 708 ( corresponding to U.S. Pat. No. 5,129,496) describes the implementation of an automatic locking brake by suitably controlling a braking system. For that, when the brake pedal is actuated and when the vehicle fails to meet a very small speed value again, the braking pressure is locked up in at least one wheel brake, i.e., maintained constant, by switching at least one valve, and is possibly additionally increased by actuating a pressure-generating arrangement. The locked-up braking pressure is reduced again only when a desire of the driver to start off from rest is detected.
German Published Patent Application No. 196 21 628 describes an automatic locking brake function (=hillholder function) which is activated when the vehicle comes to a standstill while the service brake is actuated. The then prevailing braking pressure or braking force is maintained or built up at least one wheel brake independently of the degree of the actuation of the brake pedal, and is reduced again when the brake pedal is released. Such a hillholder function avoids extensive safety measures. To assist in starting off, for example, on uphill grades while the brake pedal and the clutch pedal are actuated concurrently, the clutch pedal takes over the activation or deactivation of the hillholder function in response to releasing the brake pedal. In systems without clutch pedals capable of being actuated by the driver, for example, in the case of automatic transmissions, no clutch pedal information exists so that the known method is not usable universally.
An object of the present invention is to specify measures for an automatic locking brake function which provides a starting-off aid without complex sensor mechanism and which is usable universally at the same time.
An automatic locking brake function or a hillholder function which maintains the braking force (locks up pressure) when the braking pedal is actuated and the vehicle is detected to stand still, which releases the braking force again in response to releasing the brake pedal (preferably, the function remains active for a certain time subsequent to releasing the pedal; after the time has elapsed or in response to detecting a desire to start off from rest, the braking force is released again), a universally usable starting-off aid is carried out, in particular on uphill grades. Advantageously, the vehicle does not roll backward. The driver receives a clearly reproducible behavior in which the release of the service brake, at intermediate term, results in an unbraked vehicle in any case, as the driver is used in vehicles without this additional electronic function as well.
In a particularly expedient manner, a misuse of the hillholder function as parking brake is not possible.
Furthermore, it is particularly advantageous that in a hydraulic braking system, only relatively low demands need to be placed on the pressure-holding capability of the valve which locks up the pressure. This is because the time interval during which the pressure differential is present across the valve is limited.
It is particular beneficial that it is guaranteed that, while the function is activated, the driver is ready and therefore able to take over the holding of the vehicle in the case of a fault.
It is particular expedient that the hillholder function makes do without complex sensor mechanism such as a driver-presence detection, and that it is usable universally, also with automated transmissions.
The mentioned conditions for activating and deactivating the hillholder function are advantageously accompanied by at least one further condition which improves the performance reliability of the hillholder in an advantageous manner. The specific period of time for which the maintaining of the braking force is extended while the accelerator pedal is actuated and the brake pedal is released, is rated in such a manner that, on uphill grades, the vehicle is held for as long as the maintained braking force is needed for starting off from rest. In this process, in an exemplary embodiment, a slight reduction in braking force or braking pressure is advantageously initiated already during this period of time. In the following, the latter will also be understood as xe2x80x9cmaintaining the braking forcexe2x80x9d or xe2x80x9clocking up the braking pressurexe2x80x9d.
By the described conditions for activating or deactivating the hillholder, it is expediently guaranteed that the vehicle is automatically immobilized only if the driver is ready, i.e., if the driver has not left the vehicle. This is also true in connection with an automated transmission where clutch and/or gear information does not provide any information on the presence of the driver.